Polyisocyanate mixtures containing methylene bis(phenylisocyanate) and higher functional methylene-bridged polyphenyl polyisocyanates are known. It is also known that the amount of the various isomers of methylene bis(phenylisocyanate) can vary over a wide range. For example, U.S. Pat. No. 3,344,162 describes the production of aromatic polyisocyanates containing from about 50 to about 95 percent by weight of the diisocyanate, of which from about 10 to about 95 percent by weight is the 2,4'-isomer. U.S. Pat. No. 3,362,979 describes a polyisocyanate mixture containing from 20 to 100% by weight of the diisocyanate, of which 20 to 95 percent by weight is the 2,4'-isomer. U.S. Pat. No. 3,676,497 describes the preparation of an amine mixture (which can subsequently be phosgenated to prepare the corresponding isocyanate) containing diamines and higher functional amines, where the diamine portion consists of from about 15 to about 85 percent by weight of the 4,4'-isomer, from about 10 to about 60 percent by weight of the 2,4' -isomer, and from about 5 to about 25 percent by weight of the 2,2'-isomer.
U.S. Pat. No. 3,960,788 describes the use of a polymethylene polyphenyl polyisocyanate in the production of a rigid isocyanurate foam. The isocyanate used had an average functionality of from about 2.1 to about 2.4 and consisted of a mixture of
(i) from about 60 to about 75% by weight of methylene diphenylisocyanate, said methylene diphenylisocyanate having an isomer content as follows: PA1 (ii) from about 25 to about 40% by weight of higher functionality methylene-bridged polyphenylpolyisocyanates.
(a) from about 60 to about 90% by weight of the 4,4'-isomer, PA2 (b) from about 18 to about 33% by weight of the 2,4'-isomer, and PA2 (c) from about 2 to about 7% by weight of the 2,2'-isomer, and
The preferred isocyanate had a diisocyanate content of about 67% with the 4,4'-, 2,4'- and 2,2'-isomer contents being about 74%, about 22%, and about 4% respectively.
Additionally, several patents describes various uses for high 2,4'-isomer content isocyanates. See, e.g., U.S. Pat. Nos. 4,113,014, 4,162,357, 4,163,095, 4,261,852 and 4,448,904.
U.S. Pat. No. 4,256,849 describes the use of a mixture of isocyanates based on methylene bis(phenylisocyanate) to produce flexible polyurethane foams. The isocyanate mixture is described as having a 4,4'-diisocyanate content of from 60 to 90% by weight, a 2,4'-diisocyanate content of from 3 to 30% by weight, and a higher functional methylene bridged isocyanate content of from 0 to 37% by weight. Finally, a variety of other isocyanate mixtures based on methylene-bis(phenylisocyanate) have been described in the art. See, e.g., U.S. Pat. Nos. 4,237,240, 4,251,639 and 4,365,025.
While foams prepared from the isocyanate mixtures of the prior art are satisfactory for many applications, such foams may be too firm for other applications. Additionally, the processing of the foams of the prior art in many cases is not acceptable.